The present invention relates to a device for testing the vibration strength of a test body. The device comprises holding elements to removably hold the test body, with at least one of the holding elements being mounted in a way which allows it to vibrate. The device comprises an electromagnetic oscillation exciter with two parts that are movable in relation to one another for exciting vibrations. These two parts of the oscillation exciter are bridged by at least one elastic member. This elastic member is dimensioned in such a manner that it limits the displacements of the two oscillation exciter parts relative to one another. Therefore, in all foreseen operating conditions an air gap remains between the said two oscillation exciter parts permitting excitation with good efficiency.
A known device for a fatigue vibration strength test of test bodies, consists of a framework with elastic feet. The framework has a base standing on elastic feet. On the top side of the base there is a holding element, to hold the lower end of, for instance, a rod shaped, vertical test body. At the top of the framework there is a vertical threaded spindle which can be raised or lowered vertically. The lower end of the spindle is connected to a vibration body, which is also designated as the main moving mass, by means of a uncoupling pre-load spring, the former being provided with a holding element to hold the upper end of the test body. An electro-magnetic oscillation exciter has a first part with a magnet yoke, a magnetic core and a coil, which are also connected to the lower end of the threaded spindle; and a second part, i.e. an armature, which is movable in relation to the first part. The armature is relatively vertically adjustable with respect to the vibration body by manually operable adjustment means. There is also a transducer for the purpose of measuring the forces imparted to the test body. The transducer and the coil of the oscillation exciter are connected to the electronic circuits.
When testing the test body, it is made to vibrate, whereby the vibration body, which is connected to the upper end of the test body, also vibrates quite strongly. Moreover, the base connected to the lower end of the test body and the framework parts firmly connected to it, also vibrate a little. The pre-load uncoupling spring transmits only a small part of the vibration load from the vibration body to the threaded spindle, and therefore uncouples the vibrations of the vibration body from the threaded spindle. Hereafter, the totality of the mechanically vibrating parts will be called the oscillator. The excitation of vibrations is controlled by the electronic part in such a manner, that the oscillator vibrates at its natural vibration rate, so that during the test, adjustable vibrational or alternative forces are produced by the electronics. By adjusting the threaded spindle it is possible to superimpose via the pre-load uncoupling spring a static push or pull force on the alternating forces.
To allow the oscillation exciter to work, the air gap between the armature and the fixed part must be, on the one hand, wide enough to prevent the armature, when vibrating, to touch the fixed part of the oscillation exciter, and, on the other hand, it should not be so wide that the performance and efficiency of the exciter is adversely affected. However, the stroke of the vibrating armature depends on the required vibrating force and the rigidity of the test body. To allow the armature to vibrate freely, it is therefore sometimes necessary to widen the gap to such an extent that the efficiency is impaired. This of course is a considerable disadvantage. If one wants to always work with the different test bodies with the highest efficiency, it will be frequently necessary to alter the width of the gap by means of the already mentioned means of adjustement, a process that calls for additional work.
If on the occasion of a fatigue vibration strength test one adjusts a definite static push or pull force by means of the threaded spindle, the distance of the armature to the fixed part of the oscillation exciter also changes. When altering the static force, one is therefore often compelled to change the distance of the armature from the vibration body.
When carrying out a fatigue vibration strength test, the resonance of the oscillator is determined by the masses and elasticity of the various vibrating parts. Usually, the mass of the test body is small in comparison to the total mass of the oscillator, and the test body is also the part of the oscillator which is easiest to deform. If one imagines a oscillator, for simplicity's sake, built from a spring and a vibrating mass, the function of the spring is executed mainly by the test body, while the mass is about equal to the mass of the vibrating body. Correspondingly, the resonance frequency is also to a large extent determined by the spring-rate or -stiffness of the test body. For an oscillator consisting of a spring and a mass vibrating along a straight line, the resonance frequency is proportional to the square root of spring stiffness. The frequency therefore rises with the growing spring stiffness of the test body.
But it is also possible that other resonance vibrations with other vibration modes occur. In particular, the framework and other parts of the device generate natural frequencies, whose major parts are at least above a certain boundary frequency, which depends on the construction of the device and lies typically in the range of 300 Hz. One endeavours, therefore, as much as possible, to execute fatigue vibrating strength tests with a frequency below the above-mentioned boundary frequency. If the test is carried out with high frequencies, this may cause the test body to heat up due to the lost heat set free in it, a phenomenon that is generally undesirable. For this reason too one is inclined to keep the vibrating frequency low. But, as already mentioned, the resonance frequency of the oscillator depends to a large extent on the spring stiffness of the test body. For this reason one is often compelled to carry out tests with frequencies above the aforementioned boundary frequency, and this is also a serious disadvantage.